Electronic direct pressure control of a motor vehicle automatic transmission requires the use of several solenoid operated pressure control actuators. Each actuator controls the supply of fluid pressure to a torque transmitting device (hereinafter clutch) for both nonshifting and shifting modes of operation.
During the nonshifting or steady state mode of operation, one or more clutches are maintained fully engaged and the remaining clutches disengaged. The transmission operating pressure (hereinafter line pressure) is regulated in relation to the torque to be transmitted, and the solenoid coils of the actuators associated with the active or engaged clutches are energized to provide substantially full communication between such clutches and the source of line pressure. The voltage applied to the coils in such instance may be intermittent (pulse-width-modulated) to minimize power consumption and heating, but the actuators are intended to remain fully open or fully closed as the case may be.
During shifting modes of operation, one or more clutches are being progressively engaged (on-coming) or disengaged (off-going). The transmission line pressure is still being regulated in relation to the torque to be transmitted, and the solenoid coils of the actuators associated with the on-coming and off-going clutches are intermittently energized (pulse-width-modulated) to alternately open and close the actuator supply ports for alternately supplying fluid pressure to and exhausting fluid pressure from the respective clutches. In this way, the engagement pressures of the on-coming and off-going clutches are variable according to a predetermined schedule substantially between zero and full line pressure.
To achieve the operation described above, different driver circuits are required for the nonshifting and shifting modes of operation.
In the nonshifting mode of operation, the actuators are maintained in a predetermined position (fully open or fully closed), and the drive circuit should reduce the applied voltage to minimize power consumption and heating. To reduce switching losses, this is typically achieved by pulse-width-modulating the applied voltage. However, the magnetic force of the actuator is directly proportional to the coil current and the circuit must contain elements to perpetuate the coil current between the voltage pulses in order to maintain the actuator in the desired position. This function is typically achieved by connecting a free-wheeling diode in parallel with each coil. Each time the applied voltage is interrupted, the free-wheeling diode conducts to circulate the stored energy through the coil.
In the shifting mode of operation, the actuators alternately open and close in time with the modulation of the applied voltage. This requires fast response of the actuator. From a control standpoint, this means that the coil current must be quickly reduced to zero each time the applied voltage is interrupted. However, quick interruption of the coil current results in a potentially damaging inductive voltage spike and a snubber circuit is typically employed to protect the switching device. A representative snubber circuit for such applications may comprise a Zener diode connected in parallel with the current carrying terminals of the switching device; the Zener diode in such instance limits the transient voltage across the switching device to a safe value.